The present invention relates to a method for determination of surface texture.
The visual appearance of injection moulded components are of great importance for the customer apprehension of the total product quality. When different suppliers are employed for the supply of sub-assemblies for e. g. mobile phones, such as batteries and front covers, problems occasionally arise when the parts are assembled. In large scale production it is necessary to provide a number of manufacturing tools and often to employ several different toolmakers in order to maintain the desired volumes. A product, e. g. a mobile phone, assembled by several different parts supplied from different manufacturers always differs from each other with regard to surface texture resulting in poor fit, mismatching colour, and gloss, etc.
In the mechanical industry, roughness metrology has been employed for several years as a method for quantitative specification and measurement of tools. The presently used surface topography specification is based on comparisons of e. g. polymer masters which should meet the quantitative requirements of the German VDI scale of surface textures (VDI 3400: June 1975). This scale is a logarithmic scaling of peak-to-valley and average-height amplitudes of the surface texture as measured on tools of typically VDI 12 to 45, which indicate the average roughness. Masters are produced from the VDI specified tools and are used only for qualitative evaluations of the finished products. Other methods used today are visual master plates and comparison between parts.
However, the qualitative comparisons between polymer masters and final products give a total tolerance range which is too wide and varying in order to achieve satisfactory quality control.
The specification of tools in accordance with the VDI classes results in the surface texture being specified in amplitude only, not in wavelength. This leads to an inaccurate specification of the surface texture with no quantitative possibility to pick up common injection moulding manufacturing problems like varying mould temperatures, varying pressure, and material variations. Unsatisfactory specifications do not only result in topographical variations in the end products but also in related problems-concerning varying tactile- and colour/gloss properties.
The object of the invention is to provide a solution to the above mentioned problems. This is achieved by a method for determination of surface texture in which a specified quantification of the surface texture is achieved by a combination of a maximal height of profile parameter (Rz) and a profile mean wavelength parameter (RSm).
Preferred embodiments of the invention are provided in the dependent claims.
By the invention, a method is provided using a combination of only two ISO standardised parameters. Hereby a sharp tool is received, not only able to objectively classify surface textures into a specified scale of work piece texture, but also able to detect known texture problems, such as poor replication of the die texture in the work piece due to process or material defects.
Of all the different ISO parameters available, two parameters have shown to be particularly relevant for important requirements such as colour, tactile response, and gloss. These two parameters are maximal height of profile (Rz) and profile mean wavelength (RSm). Thus, colour, tactile response, and gloss are related to the scale parameters and may be better controlled.
The design of process models makes it possible to directly determine machining parameters for individual machines for a given specification of the work piece texture, resulting in shorter lead times for the tool.
By the method of the invention, a specified quantification of the surface texture is achieved. Thus, the old and subjective method of characterisation of the surface texture is replaced. Instead an objective method for characterisation of surface texture both on a tool surface and a plastic surface of the end product is achieved.
The use of the roughness parameters in two dimensions in accordance with the method of the invention is a cheap and convenient method. The measuring equipment and the method is standardised and industrially accepted. Due to the method of the invention it is easier to buy new tools from different toolmakers and to check the tool quality before launch thereof. This is of importance for large scale production.
The method of the invention has been developed to give a standard for an objective quantifying of plastic surfaces. However, the proposed method of the invention to characterise surface textures is applicable not only to injection moulding of electro discharge machined (EDM) surfaces. It is also applicable to other mould materials, such as light metals and alloys of light metals, along with tool textures not only machined by EDM, but also etched surfaces. The light metals may be Al, Mn, Zn etc.